Stepped fluid energy mill

ABSTRACT

A fluid energy mill of the confined vortex type is provided with paraxially symmetric discontinuities projecting from each of the axial walls. These serve to reduce the high radial velocities that tend to occur near the axial walls. As a consequence, there is a reduced tendency for oversize particles to escape along the axial walls and into the product collector and product uniformities are improved.

United States Patent [1 1 Schurr STEPPED FLUID ENERGY IVIILL [75] Inventor: George A. Schurr, Newark, Del.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: Oct. 15, 1971 [21]. Appl.No.: 189,586

[52] ILS. Cl"... ..241/5, 241/39 [51] Int. Cl ..B02c 19/06 [58] Field of Search ..241/5, 39

[56] References Cited 1 UNITEDISTATES PATENTS 2,191,095 2/1940 ,Hobbie. ..'241/39 1 Apr. 10, 1973 Primary Examiner-Granville Y. Custer, Jr. Attorney-Donald A. Hoes 7] ABSTRACT A fluid energy mill of the confined vortex type is provided with paraxially symmetric discontinuities projecting from each of the axial walls. These serve to reduce the high radial velocities that tend to occur near the axial walls. As a consequence, there is a reduced tendency for oversize particles to escape along the axial walls and into the product collector and product uniformities are improved.

7 China, Drawing Figures 10/1954 Chatelain ..241/39 Fay ..241/5 PATENTEDAFRIOISYS 72 4 4 FIGJ I/f 4 l2 5 711/8 l2 bllllllrl INVENTOR GEORGE A. SCHURR STEPPED FLUID ENERGY MILL BACKGROUND OF THE INVENTION Fluid energy mills of the confined vortex type are well known and widely employed in certain industries such as the pigment, cosmetic and plastic industries because of their efficiency and economy in comminution of particulate solids. A number of early designs are described in considerable detail in U.S. Pat. No. 2,032,827.

Most fluid energy mills are variations on a basic configuration of a disc-shaped chamber enclosed by two generally parallel circular plates defining axial walls and an annular rim defining a peripheral wall, the axial length or height of the chamber being substantially less than the diameter. Around the circumference of the mill are located a number of uniformly spaced jets for injecting the gaseous fluid which furnishes the energy for comminution, along with one or more injectors for feeding the particulate solids to be comminuted. The jets are oriented such that the gaseous fluid and particulate solids are injected tangentially to the circumference of a circle smaller than the chamber circumference. A conduit coaxial to and in direct communication with the grinding chamber is provided for discharge of the comminuted solids to a cyclone or bag filter for collection.

Fluid energy mills combine both grinding and classification within a single chamber, and the fluid mechanical principles governing these two processes have been described in the literature. As the gaseous fluid is fed tangentially into the periphery of the chamber along with the solids to be comminuted, a vortex is created wherebythe particles are swept along a spiral path to be eventually discharged at the central outlet. The passage of the fluid conveying the particles can be resolved into a tangential component of velocity, V which is a measure of the centrifugal force acting on the particle tending to. keep it at the outer periphery of the chamber, and the radial component of velocity, V,., which is a measure of the dragforce generated by the action of the fluid against the particle tending to force the particle towards the central discharge opening. By proper selection of conditions, such as rate and tangency of fluid injection, these opposing forces can be adjusted such that particles above aspecific size tend to be kept within the mill until sufficient attrition occurs, both by collision with other particles and the chamber walls, to reduce them to the desired sizes, whereupon the drag forces become dominant over centrifugal forces and the particles are swept into the central discharge zone.

Fluid energy mills are best adapted to comminution of accretions or aggregates of single particles. Although they are generally recognized to be unexcelled for this purpose, nevertheless it has been observed that considerably more material of undesirably large particle sizes frequently escapes into the product than would be expected on the basis of the calculated cut size for the particular conditions prevailing during milling. To reduce the amount of oversized material, it has been necessary to increase the intensity of grinding by reducing the solids feed rate and increasing the fluid to solids ratio with consequent greater costs in terms of reduced capacity per mill. In many cases in the titanium dioxide pigments industry this also results in what is termed over grinding of the pigment with adverse effects being noted in color and chalking resistance. Although various modifications heretofore have been proposed aimed at preventing passage of this unwanted, oversized material into the product, none has proven to be wholly satisfactory.

SUMMARY OF THE INVENTION My invention has developed from a detailed investigation of the fluid mechanics of fluid energy mills of the confined vortex type. Heretofore it was generally considered that in operation under a given set of conditions, simple rotational flow occurs such that for a given radial position within the vortex chamber, the paraxial distributions of both the tangential component of velocity, V,, and the radial component of velocity, V,, are uniform. My experimentation has shown, however, that whereas this is true for V,, it is nottrue for V,, with much higher than average radial velocities being found near the axial walls of the chamber. The magnitude of these deviations are found to depend on the ratio of the tangential velocity to the average radial velocity (V,) calculated for an assumed uniform flow profile. For V,/V, 4, the maximum radial velocity at the wall is about four times the average value, increasing to seven times the average value as V,/V, approaches 10. At the values of V,/V, near 20 the maximum radial velocity at the boundary layer near the axial walls increases to ten times the average value. These regions of high radial velocities afford a mechanism whereby oversized particles are able to escape from the mill into the product collector before they are reduced to the desired size. Calculations show that such high radial velocities at the boundary layer near the axial walls will permit discharge of particles an order of magnitude larger than would be possible if paraxially uniform radial velocities prevailed.

In accordance with my invention this deficiency of vortex-type fluid energy mills is alleviated by providing discontinuities in'the axial walls. More specifically, I have found that by providing paraxially symmetric discontinuities projecting from the axial walls of the chamber, there is a reduction in the radial velocity near those walls such that substantially improved classification and grinding functions are achieved.

The term discontinuity is used herein in its accepted fluid flow sense to define intersecting surfaces, i.e. as opposed to curved surfaces, over which a gas cannot flow without creating at least some small boundary zone of reduced pressure. Advantageously the discontinuities in the axial walls of the fluid energy mill of the invention will be abruptly divergent, i.e. stepshaped-in cross-section as would be defined by surfaces intersecting at less than The discontinuities, which are concentric of the chamber axis, should be located at a distance of 0.86-0.50 Rtherefrom, most preferably'at about 0.70 to 0.80 R, where R is the radius of the chamber as measured from the cylindrical axis of the chamber to the periphery. The magnitude of each of the projections from the axial walls has been found to be of little im portance but in general it is preferred that together they effect a change in chamber height by about 5 to 50 percent. Normally a projection of at least is inch from each axial wall (1/8 inch total) is desired, but a preferred minimum is 0.10 inch to give a total chang in paraxial height of at least 0.20 inch. Largely because of the greatly improved classification of particles obtained thereby, the invention makes it possible to achieve a superior product with respect to smallerparticles and narrower size distributions than prior art methods. A further advantage of the invention is that the improved comminution can be accomplished without unnecessarily restricting fluid flow and thus without reducing grinding rates or causing pluggage.

DETAILED DESCRIPTION OF THE DRAWINGS The invention will be further described with reference to the drawings, not to scale and with the same reference characters used to denote identical parts, wherein:

FIG. 1 shows in vertical cross-section an apparatus of the invention,

FIG. 2 is a horizontal cross-section of the device of FIG. 1 taken normal to the axis at the level of the inlet jets,

FIGS. 3, 4, 5, 6 and 7 illustrate in cross-sectional elevational views, vortex chambers having discontinuities of various shapes.

In FIG. 1 and FIG. 2, 1 is a source of fluid, which in the case of superheated steam has temperatureand pressure-controlling capabilities. Afluid header 2 encircles the peripheral wall 4 of circular grinding chamber 5. The nozzles 3, of which only four are shown, interconnect the header and the grinding chamber. The wall of the cylindrical discharge port 6 and the exhaust duct 7 are axially located. Each nozzle, 3, enters the peripheral wall 4 of the chamber at an angle such that the extension of the nozzle axis is tan-' gent to a circle about the center of the chamber which has a radius smaller than the radius, R, of the chamber; A multiplicity of these nozzles is advantageously used, l6being convenient for a chamber of 36 inches diameter. The chamber is shown to be relatively disc shaped, its actual dimensions being determined by the upper and lower circular plates 8 and 9, peripheral wall or rim 4, and the pair of identical upper and lower rings ll defining opposing, concentric, symmetrical discontinuities 12. A venturi feeding device 10 serves to introduce the solid material to be ground to the chamber, it being aligned somewhat tangentially to facilitate flow of the solids and fluid into the chamber vortex. The cylindrical discharge opening formed by discharge port 6, in conjunction with the conical enclosure 13, forms a centrifugal separator into which the ground product settles while the fluid flows out through exhaust duct 7.

Where the mill is to be used for comminuting hard, crystalline,'inorganic materials such asrutile or anatase titanium dioxide, the chamber should be provided with suitably shaped liners of hardened alloy or refractory carbides.

As described above, the discontinuities, or steps, are located at a distance of between 0.86 and 0.50 Rfrom the vortical or chamber axis where R is, as depicted in FIG. 3, the radius of the chamber measured from the central axis of the periphery. The optimum location of the steps will depend somewhat on the geometry of the grinding chamber, the size of the discharge outlet and the rates atwhich fluid and solids are to be charged.

The preferred location in most cases is near 0.75 R,

that is about 0.70 to 0.80 R, since it appears that the thickness of the boundary layers near the axial walls build to a maximum at or near this point. In certain instances, more than one discontinuity in each axial wall may be advantageous, as illustrated in FIG. 5.

The vortex chamber shown in FIG. 3 will be seen to have a paraxial height, h, two discontinuitieslZ which are triangular in cross-section and each of a height, y. The discontinuities are separated by a distance, x, which is equal to 2y and which should be about 5 to 50 percent of h. The angle of the discontinuity will be seen to be about In FIG. 4 the discontinuities constitute 90 steps. In FIG. 5 each discontinuity is formed by a pair of abrupt steps. FIGS. 6 and 7 represent still other embodiments.

Regardless of the shape of the discontinuities, the axial walls may be relatively planar or may be converging as, for example, is described in U.S. Pat. No. 3,462,086.

It will be noted that in the drawings discharge ports 6 and 7, both concentrically located around the vertical axis, enable the comminuted product to discharge in one direction to a separator, whereas, the gaseous fluid is discharged in the opposite direction. This is the preferred arrangement where inorganic pigments are comminuted and the gaseous fluid is steam. In other instances, particularly where air is the gaseous fluid, product and gas may be discharged through a single large conduit, located on either wall, to cyclones or bag filters.

DESCRIPTION OF SPECIFIC EXAMPLES General The specific examples I through V hereinafter utilize a fluid energy mill having a chamber that is 8 inches in diameter with a maximum height of 1 inch, a series of seven tangential ring jets for injecting gases at supersonic velocities, and one tangential solids injector of the venturi type,,said jets being equi-spaced on the periphery of the mill. Variations in axial wall discontinuities are effected by using interchangeable mill heads and bottom plates of specific configurations.

Where reference is madeto a Control, it will be understood that this involves, for comparative purposes, the use of a fluid energy mill of the prior art under otherwise identical processing conditions. The

specific prior art mill employed is the same as that described in the preceding paragraph except that in place of the discontinuities there are used smooth, converging axial walls as described in U.S. Pat. No. 3,462,086. In particular the walls converge at a 6 angle for a maximum height of 1 inch at the periphery to terminate in a 4 inch discharge zone. A converging wall mill of this design has previously been considered to be outstanding in terms of its ability to provide a product of highly uniform particle size.

EXAMPLE I The particulate solid employed in Profax polypropylene powder manufactured by Hercules, Inc. It is composed of agglomerates of individual particles of 0.08 microns average diameter, the agglomerates having sizes, as measured by screening tests, percent by weight greater than 74 microns and percent by weight greater than 37 microns.

Air at 75F. and 100 p.s.i.g. is used as the source of energy for the vortex and for feeding the polypropylene powder. The air feed rate is 100 SCFM. The venturi type feed injector is fed by a vibrating feeder delivering solids with a i 2 percent by weight accuracy. Both gas and comminuted solids are discharged from an upwardly directed central conduit of 4 inches diameter into a filter bag measuring 50 ft. in area. Particle size distributions of the comminuted products are determined by a Laboratory Jet Sieve manufactured by the Alpine Corporation.

The mill design corresponds generally to that described in connection with FIGS. 1, 2 and 3. Thus axial end plates are utilized to give a peripheral grinding section converging inwardly at a 14 angle for a radial distance of one inch from the periphery. At this point (0.75 R) a A inch step is provided in each axial wall to give a total increase in paraxial height of onehalfinch, i.e., h is one inch.

Processing at a rate of 50 lbs/hr. polypropylene powder under the conditions described leads to a comminuted powder in which 99 percent by weight is less than 74 microns in size whereas 84 percent by weight is less than 34 microns in size. At a solids feed rate of 25 lbs./hr., the product contains 99.6 percent by weight less than 74 microns in size, and 95.0 percent by weight less than 37 microns in size.

Particle size analysis of a comminuted Control at 50 lbs/hr. solids feed shows that only 81 percent by weight of the particles are less than 74 microns in size and only 72 percent by weight are less than 37 microns in size. When the solids feed is reduced to 25 lbs/hr. still only 90 percent by weight is less than 74 microns and only 85 percent by weight is less than 37 microns.

EXAMPLE II In this case the mill configuration corresponds to that shown in FIG. 4. The grinding section, consisting of parallel axial walls one-half inch apart, extend from the periphery inwardly for 1 inch to a point at 0.75 R where steps in each axial wall provide a total increase of one-half inch in the paraxial height of the chamber.

The axial walls then continue parallel to one another until intersecting with the exit port.

Feeding the polypropylene powder, at 50 lbs./hr. gives a product having a screen analysis of 97 percent by weight less than 74 microns in size and 72 percent by weight less than 37 microns in size. At a feed rate of 25 lbs./hr., the product is analyzed to be 99 percent by weight less than 74 microns in size and 90 percent by weight less than 37 microns in size.

EXAMPLE III 74 microns in size and 80 percent by weight less than 37 microns in size. At a solids feed of 25 lbs./hr., 98.4 percent by weight of the product is less than 74 microns in size and 96 percent by weight is less than 37 microns in size.

The results of Examples I, II and III are summarized in the following Table. It will be readily seen that as compared to the Control, the mill devices of the invention give rise to substantial reductions in the amount of oversize material in comminuted products that are obtained.

TABLE I Example Example Example Control I II III Particle Size lbs./hr. lbs/hr. lbs/hr. lbs/hr. by Weight 50 25 50 25 50 25 50 25 %Greater than 74p. l9 l0 1 0.4 3 l t 4 1.6 37-74;; 9 5 15 4.6 25 9 16 2.4 Less than 37p. 72 84 95 72 80 96 EXAMPLE IV Powdered 2-mercaptoimidazoline, a well known accelerator for the vulcanization of neoprene is the material to be comminuted. It is composed of agglomerat'es of 2-5 micron particles. Comminution is effected in the same mill employed in Example I. The conditions are the same except that the solids feed is 43 lbs/hr. The ground product is found, by wet screen analysis, to contain 94.3 percent by weight less than 20 microns in size. A correspondingly produced Control product has only 81.8 percent by weight less than 20 microns in size.

EXAMPLE V In this case the mill is of the same dimensions and configuration as that described in Example I, except that product is discharged from a 5 inch diameter conduit attached to the bottom plate and fluid escapes through a smaller conduit attached to the upper plate.

The powder to be comminuted is a high gloss rutile TiO pigment. The fluid supplied to the seven ring jets and one feed jet is superheated steam at 128 p.s.i.g., 400C. While maintaining a constant steam flow of 320 lbs./hr., the TiO is fed to the mill at various rates ranging from 5.2 to 1.3 lbs. steam/lb. TiO (62-250 lbs. TiO per hour), taking precautions that uniform conditions prevail at each feed rate before taking samples.

- The resultant pigments are, as compared toa Control,

means for charging pulverulent solids to an outer portion of the chamber, and discharge means for withdrawing pulverulent solids and gaseous fluid along the axis of the chamber, the improvement wherein at least onegenerally ringshaped projection, concentric of the chamber axis and located a distance of about 0.50 R to 0.86 R

therefrom, extends from each of said opposing circular walls into the chamber to provide spaced symmetrical discontinuities.

2. Mill according to claim 1 wherein the discontinuities are defined by surfaces intersecting at an angle of less than 135.

3. Mill according to claim 2 wherein the discontinuities are defined by surfaces intersecting at an angle of less than 90.

4. Mill according to claim 1 wherein the discontinuities are located a distance of about 0.70 to 0.80 R from the chamber axis.

5. In a process for comminuting a material comprising pulverulent solids wherein said material is subjected to fluid energy milling in a gaseous vortex confined within a truncated cylindrical chamber, said vortex comprising a peripheral milling zone, an intermediate classifying zone and a central discharge zone, the improvement wherein at least one divergent discontinuity is symmetrically located on each axial wall of said chamber at a distance of between 0.86 R and 0.50 R from the vortical axis, where R is the radius of said chamber, the magnitude of the divergence of said discontinuity being such as to create at least an increase in paraxial height of said milling/zone at said discontinuities of at least 5 percent.

6. Process according to claim 5 wherein at least one discontinuity is provided for each axial wall at a distance of 0.70-0.80 R from the vortical axis.

7. Process according to claim 5 wherein the material is Tiog. 

2. Mill according to claim 1 wherein the discontinuities are defined by surfaces intersecting at an angle of less than 135*.
 3. Mill according to claim 2 wherein the discontinuities are defined by surfaces intersecting at an angle of less than 90*.
 4. Mill according to claim 1 wherein the discontinuities are located a distance of about 0.70 to 0.80 R from the chamber axis.
 5. In a process for comminuting a material comprising pulverulent solids wherein said material is subjected to fluid energy milling in a gaseous vortex confined within a truncated cylindrical chamber, said vortex comprising a peripheral milling zone, an intermediate classifying zone and a central discharge zone, the improvement wherein at least one divergent discontinuity is symmetrically located on each axial wall of said chamber at a distance of between 0.86 R and 0.50 R from the vortical axis, where R is the radius of said chamber, the magnitude of the divergence of said discontinuity being such as to create at least an increase in paraxial height of said milling/zone at said discontinuities of at least 5 percent.
 6. Process according to claim 5 wherein at least one discontinuity is provided for each axial wall at a distance of 0.70-0.80 R from the vortical axis.
 7. Process according to claim 5 wherein the material is TiO2. 